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sum<$>iterate f[2](!1)[2] f(x:y)=f y++!n=y!(1+length y<$[1n+1)++(n<$[1..x]) f e=ee!_=e
Try it online!Try it online!
The whole Levine sequence.
sum<$>iterate f[2] f(x:y)=f y++(1+length y<$[1..x]) f e=e
Try it online!
sum<$>iterate(!1)[2] (x:y)!n=y!(n+1)++(n<$[1..x]) e!_=e
